The charging of batteries should be performed as carefully as possible in order not to damage the battery or reduce its capacity and at the same time the charging should be performed as quickly as possible, these two criteria cannot always be obtained at the same time. In order to shorten the charging time, the charging current can be increased. A high charging current can however damage the battery. To this end the battery manufacturers have strict instructions on how high a charging current is allowed to be as a function of battery size. A normal recommendation is that the charging current is obtained by multiplying the capacity of the battery in ampere hours with 0, 1, ie a 20 Ah battery should be charged with 2 A.
A battery charger is therefore “locked” in a window between being too small, with long charging time, and too large, with detrimental influence on the battery life. As a consequence of this, users with a number of different battery sizes are forced to have a number of different battery chargers, alternatively they charge outside the recommendations of the manufacturers. For all charging of batteries, the final charging voltage has a large impact on the life of the battery. A too high voltage means that the battery develops gas with an increased concentration of sulphuric acid and accelerated grid corrosion as a consequence. A too low voltage means an uncompleted charging with partial sulphating and lost battery capacity as a consequence. A third parameter is that current ripple shall be kept low as it cause an increase in the battery temperature during charging with a decreased life as a consequence.
On linear chargers, ie chargers arranged with a transformer that converts the mains voltage to charging voltage, which are the predominant type for chargers, the current may be controlled in that the transformer is provided with several primary windings and the output voltage is varied by choice of primary winding. Due to this, the output voltage is altered and because the current is proportional to the voltage, the current can be affected. The drawback with this type is apparent on an unregulated linear charger because the voltage to the battery can rise to levels which are detrimental to the battery if quick charging is required and a too low voltage level if it is desired to reduce the current. Ripple is something undesirable on a charger due to the above mentioned problems. The development trend regarding battery chargers is a transfer to primary switched devices, which offer a more exact control of voltage and current and at the same time smaller dimensions. There exists today, as far as the inventors are aware, no changeable output currents on primary switched chargers.